1. Field of the Invention
This invention relates to the manufacture of flat glass and in particular relates to the making of flat glass by a method wherein the glass is floated on or supported by a pool of molten metal during its formation into a continuous sheet or ribbon of glass. In particular, this invention relates to that special area of interest of the manufacture of glass involving heating or cooling the glass to provide desired thermal conditions for forming glass of good optical flatness for a variety of end uses.
2. Description of the Prior Art
It has been known at least from the early nineteen hundreds that flat glass could be produced by floating a body or layer of glass on a pool of molten metal while cooling the glass and drawing it onto the surface of the molten metal to form a continuous ribbon or sheet of glass having desired thickness and width. More recently, such processes have become commercial reality such that most flat glass produced in the U.S. today is made by techniques involving flotation of glass.
At the time of the earliest patents relating to the manufacture of flat glass by a float method, there was already an appreciation of the need for a controlled thermal environment in which the molten glass could be formed into a dimensionally stable sheet or ribbon of glass. This is particularly evident in U.S. Pat. No. 789,911 to Hitchcock which teaches that a supporting pool or bath of molten metal may be subdivided into a plurality of adjacent smaller pools of molten metal, each in communication with those adjacent to it and each at slightly different temperature from those adjacent to it so that a longitudinal temperature gradient along the path of glass movement through the forming chamber can be maintained with the temperatures gradually dropping as glass is advanced through the chamber until the glass would be sufficiently chilled to have surfaces not readily susceptible to marking or damage prior to being removed from support by molten metal.
Since the time of the Hitchcock reference, there has been an appreciation that transverse temperature gradients, that is, temperature gradients normal to the path of glass advance through a forming chamber, may also be controlled to achieve any of a number of desired results. U.S. Pat. No. 3,843,346 to Edge and Kunkle describes in detail the desirability of maintaining rates of cooling in the initial portion of a glass forming chamber such that the central portion of an advancing layer of glass is caused to cool relatively more rapidly than the marginal portions of the advancing layer of glass caused to cool relatively more slowly than would naturally occur in a forming chamber not having particular facility for achieving such results. A conventional process simply providing for the natural cooling of margins at a greater rate than a thermally protected interior or central portion of an advancing layer of molten glass would be more susceptible to the development of optical distortion in its finished glass than a process like that of Edge and Kunkle.
U.S. Pat. No. 3,930,829 to Sensi, one of the instant applicants, discloses that dams or barriers submerged in a pool of molten metal upon which glass is to be formed may be positioned in paired angular relation to one another. The dam barriers are so arranged as to provide communication between molten metal at a central, upstream location in a pool of molten metal and a marginal, downstream location in the pool so that thermally-induced natural convection in the pool of molten metal between such dams or barriers causes heat to be transferred from the central, upstream location to the marginal, downstream location with a consequent redistribution of heat and temperature in the chamber.
Since the present invention contemplates the use of heat pipes within a glass forming chamber, it is useful to consider what a heat pipe is and how a heat pipe works, as well as some prior art references concerning the use of heat pipes and conditions under which heat pipes may be employed in industrial processes. Heat pipes are described in detail in an article entitled "Cooling With Heat Pipes" by Francis J. Lavoie in Machine Design, issue date Aug. 6, 1970. Briefly, a heat pipe is an enclosed container having a working fluid inside it, with the working fluid being selected so as to vaporize in one portion of the container with the vapors expanding and flowing to another location in the chamber where they are condensed with the condensed working fluid returning by means of capillary action or centrifugal force or some other transfer mechanism not involving pumping back to the location at which the vaporization takes place. A heat pipe therefore has many characteristics similar to that of an absorption refrigeration unit and is a device effective for receiving heat from its surroundings at the location where vaporization takes place in giving up heat to its surroundings at the location where condensation takes place.
Heat pipes have been employed in glassmaking furnaces as evidenced by U.S. Pat. No. 3,640,517 to Sendt and in conjunction to glass molds as shown in U.S. Pat. No. 3,644,110 to Sendt and U.S. Pat. No. 3,761,220 to Dirne. U.S. Pat. No. 3,607,209 to Lazaridis shows heat pipe extending into a pool of molten glass in a glassmaking furnace for the purpose of supplying heat to the glassmaking furnace.
While there have been numerous proposals and practices relating to control of heating and cooling in the glass forming chamber, there has not been provision made in conventional glass forming chambers for either redistributing the heat in the chamber without removing heat therefrom and allowing it to be wasted or without adding a net amount of heat thereto requiring the consumption of additional energy. It is therefore a purpose of the present invention to accomplish such a beneficial redistribution of heat within a glass forming chamber without consequent energy losses incidental to such redistribution of heat or with only such ancillary removal of heat as is necessary to ultimately cool the whole body of advancing glass in a glass forming chamber to provide the glass at a satisfactory temperature for removal from the forming chamber itself.